1. Field of the Invention
The present invention relates to a modulation/demodulation apparatus and a modulation/demodulation method.
2. Background Art
As a type of a signal modulation scheme, an orthogonal modulation scheme is widely known. In the orthogonal modulation scheme, two signals having a phase difference of 90 degrees are collectively transmitted as one combined signal. One signal forming the combined signal is referred to as an I component (in-phase component), and the other signal forming the combined signal is referred to as a Q component (orthogonal component).
In orthogonal modulation processing and orthogonal demodulation processing, uncorrelated errors present in an orthogonal modulation transmitter and an orthogonal demodulation receiver (IQ mismatches) may impose a problem. In the orthogonal modulation transmitter and the orthogonal demodulation receiver, the IQ mismatches have significant influence on apparatus characteristics, such as modulation accuracy of the transmitter and reception sensitivity of the receiver (see JP-A 2001-7882 (KOKAI), for example).
In a modulator-demodulator including an orthogonal modulator and an orthogonal demodulator, in order to correct an IQ mismatch, it is necessary to detect the presence of the IQ mismatch. The IQ mismatch can be detected by modulating, with the orthogonal modulator, two sine waves generated for an IQ mismatch detection which are orthogonal to each other and have equal amplitude, demodulating a modulated signal with the orthogonal demodulator, and analyzing a demodulation result (loop-back test), for example. This makes it possible to detect the IQ mismatch present in the modulator-demodulator. It is possible to correct the IQ mismatch by gradually adjusting a transmission circuit (or a transmission signal) and a reception circuit (or a reception signal) with taking into account a detection result of the IQ mismatch.
However, this method has a drawback in that an IQ mismatch that occurs in the orthogonal modulator and an IQ mismatch that occurs in the orthogonal demodulator cannot be distinguished. There is also a drawback in that, when an IQ mismatch that cancels an IQ mismatch in the orthogonal modulator is present in the orthogonal demodulator, it looks as if no IQ mismatch is present in both the orthogonal modulator and the orthogonal demodulator. Such drawbacks prevent the IQ mismatches from being correctly detected and corrected.